Downhole drilling system

ABSTRACT

A drilling system for drilling a borehole. The system comprising a drilling assembly including a drilling motor; a drill bit mounted on the drilling assembly to be rotated by the drilling motor, an axial drive mechanism connected to the drilling assembly for moving the drilling assembly through the borehole and operable to provide weight on the bit (WOB) during drilling, and a drill string connected to the drilling assembly. The drill string is a flexible conduit to convey fluid between the surface and the drilling assembly and the WOB and torque on bit (TOB) are provided downhole by the drilling assembly.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on and claims priority to GB Application No. 0722930.5, filed 23 Nov. 2007; and International Patent Application No. PCT/EP2008/009697, filed 17 Nov. 2008. The entire contents of each are herein incorporated by reference.

TECHNICAL FIELD

This invention relates to a drilling system. In particular it relates to a system for drilling downhole using a flowline.

BACKGROUND ART

Conventional drilling can be performed with coiled tubing (CTD) or with screwed pipe drilling (SPD). CTD can be described as drilling with a continuous pipe coiled onto a reel. It is associated with a downhole drilling motor that provides rotation to the bit. Most CTD drillings are performed using 2 inch (51 mm) to 2⅞ inch (73 mm) steel coiled pipe. The use of CTD drilling operations have been limited because of their limited reach in horizontal wells, poor weight on bit transfer in extended sections, high maintenance costs and the limited life of coiled tubing.

SPD uses jointed pipes which can either be rotated from the surface or have a downhole drilling motor to rotate the bit. A drilling rig is necessary to provide the weight on bit, to provide or handle torque on bit and to make up the drill string. The smallest pipes are about 2⅜ inches and are used in Through Tubing Rotary Drilling. SPD is widely used however some of its disadvantages include: pipe handling could be a major safety issue to rig crews, tripping time is very long, operating in live wells is complex and the potential of failure or damage of the connections between pipes.

In both CTD and SPD techniques, the drilling string (CT or pipes) provides the weight on bit (WOB) transfer from the surface to downhole and also provides the drilling torque. The coiled tubing and the jointed pipes are dimensioned to withstand these forces, and as such the large and powerful mechanical systems are required, i.e. CTD units or drilling rigs to deploy and operate the drilling equipment.

A number of systems are known that use tractors to move drilling tools through the borehole by pulling and/or pushing the tool and also typically provide weight on bit for the drilling process. Such tractors are described in U.S. Pat. No. 5,794,703, U.S. Pat. No. 5,954,131, U.S. Pat. No. 6,003,606, U.S. Pat. No. 6,179,055, U.S. Pat. No. 6,230,813, U.S. Pat. No. 6,142,235, U.S. Pat. No. 6,629,570, GB2388132, U.S. Pat. No. 6,629,568 and U.S. Pat. No. 6,651,747.

The present invention aims to provide a drilling system wherein the weight on bit and torque on bit is provided by the bottomhole assembly thereby enabling drilling operations to be performed without relying on CTD or SPD.

DISCLOSURE OF THE INVENTION

Accordingly a first aspect of the invention comprises drilling system for drilling a borehole comprising: a drilling assembly including a drilling motor; a drill bit mounted on the drilling assembly to be rotated by the drilling motor; an axial drive mechanism connected to the drilling assembly for moving the drilling assembly through the borehole and operable to provide weight on the bit (WOB) during drilling; and a drill string connected to the drilling assembly; wherein the drill string is a flexible conduit to convey fluid between the surface and the drilling assembly and the WOB and torque on bit (TOB) are provided downhole by the drilling assembly.

The drill string of the system does not have to be designed to withstand the forces required to transfer WOB and torque from the surface to the drill bit, as the WOB and TOB are generated downhole by the drilling assembly so no mechanical drilling effort is transmitted from the surface.

Typically the drill string is a hydraulic conduit.

Preferably the drill string is flexible hose constructed from polymer liners. Alternatively the drill string is service coiled tubing.

In one embodiment the drilling system can be electrically powered. In this case it is preferred that the electrical power can be provided by electrical cables run inside the flexible conduit. Electrical power can also be provided by clamping a cable to the conduit, or embedding the cable inside the structure of the conduit.

In another embodiment the drilling system is hydraulically powered. In this situation the system can also comprise a downhole alternator to convert the hydraulic energy to electric energy.

The drilling system can further comprising a control system for controlling the RPM and the WOB during drilling.

A second aspect of the invention comprises a method of operating a drilling operations comprising: positioning the drilling system as described above in a borehole to be drilled; operating the drilling motor and axial mechanism to provide the WOB and torque to cause the drill bit to drill ahead; and using the drilling string to direct fluid between the surface and the drilling assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a drilling system according to an embodiment of the invention;

MODE(S) FOR CARRYING OUT THE INVENTION

A drilling system according to an embodiment of the invention for drilling boreholes in underground formations is shown in FIG. 1. The system includes a downhole drilling assembly comprising a rotary drive system 10 carrying a drill bit 12. An axial drive system 14 is positioned behind the rotary drive system 10 and connected to a control section 16. The rotary drive system 10 includes a motor by which the drill bit 12 is rotated. The drilling assembly is connected to surface by the fluid flowline 18.

In use the drilling system is run into the borehole 20 until the bit 12 is at the bottom of the borehole. The axial drive mechanism is capable of pulling the flow line through the borehole to position the drilling assembly in place. Drilling proceeds by rotation of the bit 12 using the rotary drive system and advancing the bit into the formation by use of the axial drive system to provide the weight on bit. Control of both is affected by the control system 16 which in turn can be controlled from the surface or can run effectively independently. During drilling no mechanical efforts is transmitted from the surface to the drilling bit, as the downhole drilling assembly is capable of generating the weight on bit, RPM and torque etc, itself downhole.

By generating axial effort downhole by use of the tractor 14 coiled tubing or pipe string does not need to be used. Because the coiled tubing and pipe string are not required to generate weight on bit, the basic functions of the drill string are limited to:

acting as a flowline to convey the drilling fluid downhole;

acting as a retrieval line to get the bottom hole assembly out of the hole, especially when stuck; and

helping to run in hole with its pushing capacity.

This allows a conventional drill string, i.e. coiled tubing or jointed pipes, to be replaced with a simple flexible conduit, such as a flowline, whose main function is to circulate fluid between the surface and bottom of the borehole and does not need to be configured to handle the mechanical forces required to provide weight on bit.

Advantages of using the drilling system of the invention over jointed pipe drilling include faster tripping speed, continuous circulation while tripping, ability to operate (drill and trip) in pressurized wells, rig-less operation, easy implementation of cable telemetry. Advantages achieved over both jointed pipe drilling and coiled tubing drilling include: extended lateral reach, improved well placement, and very short radius drilling.

The aim of the flowline is to provide a hydraulic conduit for circulating the drilling fluid and can have tensile rating so that it can also be used to trip out or to free a sticky bottomhole assembly. However it does not need to perform the other mechanical functions that drilling strings consisting of jointed pipes or coiled tubing are also required to perform. To achieve this the flow line can take a number of configurations including:

A hose compatible with the oilfield environment. The hose may be constructed of polymers liners made of extruded plastic i.e. PTFE, PVDF, PEEK etc, which is combined with steel wire or composite reinforced structures for pressure resistance, an extruded polymer cover and tensile armors layers.

Can be made of a composite material or any other material compatible with the oilfield environment and capable of withstanding the pressures and forces involved in the operations.

Service coiled tubing as the mechanical specification in terms of axial force transfer or buckling are no longer required. Coiled tubing with a smaller diameter, than conventional coiled tubing used for drilling operations, such as service coiled tubing with a less than 1½ in (38 mm) OD can be used.

By not having to use conventional coiled tubing or jointed drill pipes to make up the drill string, the large mechanical systems associated with their deployment, i.e. CTD units and drilling rigs, are also not required. This is particular useful for off shore drilling operations where the size and weight of a spoil of coiled pipe are usually not compatible with the hoisting capacity of many platforms. If service coiled tubing is used as the flow line, then the service coiled tubing fleet can be used for drilling operations.

Instead of the drill string delivering the mechanical forces required to drill the well the downhole drilling assembly is configured to be capable of supplying the mechanical forces to the drill bit required to drill the well. in one preferred embodiment the axial drive system is preferably a push-pull tractor system capable of operating in an open hole, such as described in 04292551.8 and PCT/EP04/01167. The tractor has a number of features that allow it to operate in a drilling environment, including:

the ability to function in a flow of cuttings-laden drilling fluid and to be constructed so that cuttings do not unduly interfere with operation;

ability to operate in open hole;

accurate control of ROP with precise control of position and speed of the displacement;

accurate measurement of weight on bit;

the presence of a flow conduit for drilling fluid circulation in use.

In another embodiment the tractor is a continuous system with wheels or chains or any other driving mechanism.

Certain features of the tractor can be optimized for efficient tripping, such as a fast tractor speed (speed of moving the downhole unit through the well), and the capabilities of crawling inside casing or tubing. In order for the tractor to be useful for re-entry drilling, it also needs the ability to cross a window in the casing and to be compatible with a whipstock.

The motor can be an electrical motor provided with power by means of an electric cable which also provides a medium for a two-way high-speed telemetry between the surface and downhole systems the enabling a better control of downhole parameters. Intelligent monitoring of downhole parameters such as instantaneous torque on bit can help avoid or minimize conventional drilling problems such as stick-slip motion, nit balling, bit whirling, bit bouncing, etc.

The electric cable can be deployed along with the flowline from the surface to the control system of the drilling assembly. This can be achieved in various configurations, including:

the electric cable is pumped inside the flowline;

the electric cable is clamped to the outside of the flowline; or

the flowline is constructed with electric wires in its structure.

The electrical power supplied is then converted downhole into mechanical power at the bit via the electrical drilling motor and push/pull tractor to generate the WOB.

However in a different embodiment the downhole drilling assembly can be hydraulically powered. The downhole drilling system can be hydraulically powered and equipped with a downhole alternator to provide electric power to tool components. In this configuration, there is no need for electric lines from the surface to power the drilling assembly. The hydraulic motor is typically powered by the flow of fluid through the drill string into the hydraulic motor.

Control of the drilling system can be achieved a number of ways including from the surface using mud pulse or cable telemetry. Or the system can self-monitored, this is particular suitable when there are no electric lines from the surface to the downhole drilling assembly, for example when the drilling system is hydraulically powered. In this situation the control system of the assembly can include an embedded software program to control the progress of drilling operations.

Further changes can be made while still remaining in the scope of the invention. 

1. A drilling system for drilling a borehole comprising: a drilling assembly including a drilling motor; a drill bit mounted on the drilling assembly to be rotated by the drilling motor; an axial drive mechanism connected to the drilling assembly for moving the drilling assembly through the borehole and operable to provide weight on the bit (WOB) during drilling; and a drill string connected to the drilling assembly; wherein the drill string is a flexible conduit to convey fluid between the surface and the drilling assembly and the WOB and torque on bit (TOB) are provided downhole by the drilling assembly.
 2. A drilling system according to claim 1 wherein the drill string is a hydraulic conduit.
 3. A drilling system according to claim 1 wherein the drill string is flexible hose constructed from polymer liners
 4. A drilling system according to claim 1 wherein the drill string is service coiled tubing.
 5. A drilling system according to claim 1 wherein the drilling system is electrically powered.
 6. A drilling system according to claim 5 wherein the electrical power is provided by electrical cables run inside the flexible conduit.
 7. A drilling system according to claim 1 wherein the drilling system is hydraulically powered.
 8. A drilling system according to claim 7 further comprising a downhole alternator.
 9. A drilling system according to claim 1 further comprising a control system for controlling the RPM and the WOB during drilling.
 10. A method of operating a drilling operations comprising: positioning the drilling system as claimed in claim 1 in a borehole to be drilled; operating the drilling motor and axial mechanism to provide WOB and torque to cause the drill bit to drill ahead; and using the drilling string to direct fluid between the surface and the drilling assembly. 